The present invention relates to flowmeters and flow sensors of the type utilizing a fluid oscillator so arranged that the frequency of oscillation is indicative of the flow rate being measured or indicated.
For flowmeter applications where long life, little wear and insensitivity to adverse environmental influences such as excessive vibration are particularly important, efforts have been made to develop reliable and accurate flowmeters with no moving parts. Flowmeters of this type have been proposed using fluid amplifiers with negative feedback connections which cause the amplifier to oscillate at a frequency indicative of flow rate.
Heretofore such no-moving part flowmeters have utilized fluid amplifiers of the digital or bistable type which operate on jet interaction and wall-attachment principles. In these amplifiers, a power jet directed between two side walls will attach to one wall with positive feedback due to boundary layer effects until forcibly displaced to the other side wall by a control jet interacting with the power jet. By using a pair of fluid receivers and a pair of control jets on opposite sides of the power jet and interconnecting the receivers and control jets by negative feedback connections, a flip-flop, discontinuous type of oscillation is obtained. With the power jet connected in circuit with the fluid flow being measured, the oscillation frequency will vary with flow rate and is a result of interaction of the positive and negative feedback forces applied to the power jet. Flowmeters using such bistable, digital amplifiers are shown, for example, in U.S. Pat. Nos. 3,238,960-- Hatch, Jr.; 3,640,133--Adams; and 3,855,859 --Adams.
A problem with no-moving-part flowmeters of the prior art type has been that they have a limited range of operation so that their use is precluded for many applications. The reason for this is that the wall attachment effect on which the fluid amplifier depends for positive feedback effect operates only for conditions of turbulent flow. Expressed in terms of Reynolds number, which is used in fluid dynamics to indicate, inter alia, a transition point between turbulent and laminar flow, these flowmeters have no useful linear output for flow rates below that for which the Reynolds number defining the flow condition is less than approximately 2000.
Accordingly, it is an object of the invention to provide a flowmeter or flow sensor of the fluid oscillator type which is operative for laminar as well as turbulent flow conditions and thus has a greater useful operating range than has been possible heretofore.
A further object of the invention is to provide a flowmeter or flow sensor of the fluid oscillator type having a lower limit of operation expressed in terms of Reynolds number that is of the order of 400 or less and thus lower than has been heretofore possible by a factor at least 5 to 1.
A still further object is to provide a flowmeter or flow sensor of the fluid oscillator type which has improved linearity and hence better accuracy over an extended flow range.
Another object of the invention is to provide a flowmeter of the fluid oscillator type in which the amplifier element has a proportional rather than a bistable input-output characteristic so that multiple amplifier stages may be used further to extend the useful flow measurement range.
Further objects and advantages of the invention will become apparent as the following description proceeds.